Cathode-ray device



April 13, 1954 R w SEARS 2,675,499

CATHODE-RAY DEVICE Filed July 10. 1948 2 Sheets-Sheet 2 1.9 [2 I3 I l4 3If 27 OUTPUT ri i U I i as- T g -za PULSING CIRCUIT PULSIIIC CIRCUIT"/RCUIT INVENT'OR R. W SEARS 8! ATTORNEY Patented Apr. 13, 1954 UNITEDSTATES PATENT OFFICE CATHODE-RAY DEVICE Application July 10, 1948,Serial No. 38,125

9 Claims. (Cl. 315-12) This invention relates to electron dischargeapparatus and more particularly to cathode ray devices of the typecommonly referred to as storage tubes, wherein an input signal is storedin the form of a charge distribution for a period of time and convertedinto an output signal at a subsequent period.

Cathode ray devices of the storage type comprise generally, in one form,a target such as a dielectric sheet having a conductive member orelectrode in contact with one face thereof, an electron gun forprojecting a concentrated electron stream against the other face of thesheet, and a barrier grid, to which the input signal may be applied,adjacent the latter face. a device, the beam is deflected in twocoordinate directions, for example, repeatedly swept in one directionand selectively deflected in the other direction. The operationinvolves, basically, two

periods or cycles, on store and the other remove. During the store cycleor period, the potential or charge upon elemental areas of the bombardedfac of the dielectric is varied in accordance with the amplitude of theinput signal,

the charge change on each area being proportion- 9..

al to the signal amplitude at the time the beam impinges upon that area.During the remove period or cycle, the charges upon these areas areresolved into respective potential changes in an output circuitconnected to the conductive member or electrode in contact with thedielectric sheet.

Fundamentally, the charging and discharging of the elemental areas abovenoted results from emission of secondary electrons from an area when itis struck by the electron beam. For positive input signals during thestore period, secondary electrons flow from the area to th barrier gridso that the potential of the area increases in the positive sense andapproaches that of the barrier grid at that time. During the removeperiod, the barrier grid is at a constant potential, for example Zero,and the secondary current from the area is less than the primary currentthereto. Consequently the potential of the area decreases and approachesthat of the barrier grid. This potential decrease is resolved into theoutput signal. For negative input signals during the store period, thecharging and discharging takes place With polarity opposite to thatabove mentioned.

It has been found that the output signal level in devices of the typedescribed is relatively low and further that the output signals do notconform with highfidelityto th input signals. The

In the operation of such 3.

reasons for this will be analyzed in some detail hereinafter. However,it may be noted for present purposes that the low level and poorresolution may be ascribed to a secondary electron redistribution on thebombarded face of the dielectric due to a space charge or secondaryelectron spray at this surface, associated with the impinging primaryelectron beam.

One general object of this invention is to improve the perfcrmance ofcathode ray devices of the storage type. More specifically, objects ofthis invention are to increase the output level and to improve theresolution of such devices.

In accordance with one feature of this invention, means are provided forreducing undesired secondary electron redistribution on the bombardedface of the target.

In accordance with a more specific feature of this invention, means areprovided for affecting the primary electron beam so that only spacedelemental areas of the dielectric target are struck by the primary beam,these elemental areas being the same for both the store and removesweeps of the beam and so spaced that each area is beyond the region ofimpact of the secondary electron spray resulting from impingement of thebeam upon the next adjacent area or areas.

In one illustrative embodiment of the invention, the beam afiectingmeans comprises an auxiliary electrode between the electron gun and thebarrier grid and having a plurality of parallel Wires Opposite thebarrier grid and extending at an angle, for example a right angle, tothe direction of th beam sweep.

In another illustrative embodiment of this invention, the beamafijectingmean comprises a control electrode which is energized to repeatedlyblank the beam during each store and remove period or cycle, wherebyduring both cycles the beam strikes only spaced elemental areas of thetarget. 7

In both of the illustrative embodiments above noted, in effect the beamis pulsed at the bombarded face of the target. Because of the spacing ofthe elemental areas of this face, secondary electrons emanating from anyarea cannot reach an adjacent area to alter the charge thereon. Inasmuchas the regions between the spaced elemental areas are not struck by theprimary beam, these regions do not emit secondary electrons which mightaffect the bombarded areas. Thus, deleterious secondary electrondistribution with consequent altering of the desired charges uponthebom'barded areas is prevented.

The invention and the above-noted and other features thereof will beunderstood more clearly and fully from the following detaileddescription with reference to the accompanying drawing, in which:

Fig. 1 is in part a diagram of a cathode ray tube and in part a circuitschematic illustrating one embodiment of this invention;

Fig. 2 is a diagram, to an enlarged scale, of a portion of the targetand barrier grid structure included in the tube shown in Fig. 1, withvarious 1 capacitances between elements of this structure indicated;

Fig. 3 is a diagram representative of the equivalent circuit of thestructure illustrated in Fig. 2, together with the input and outputresistors;

Figs. 4A and 4B are diagrams which will be referred to hereinafter inthe discussion of certain phenomena involved in devices of the type towhich this invention pertains;

' Figs. 5A to 5D are graphs illustrating the operation of a deviceembodying this invention;

Fig. 6 is in part a diagram of a cathode ray tube and in part a circuitschematic illustrating another embodiment of this invention;

Fig. '7 is a diagram showing a two-sided storage tube embodying featuresof this invention; and

Fig. 8 is a diagram and schematic, similar to Figs. 1 and 6, showinganother illustrative embodiment of this invention.

Referring now to the drawing, the cathode ray tube illustrated in Fig. 1comprises an evacuated enclosing vessel ll) having at one end thereof anelectron gun which includes a cathode ii,

a control electrode l2, anodes l3 and I4 and a focussing electrode 15.The electron gun produces a concentrated electron beam which isprojected centrally between two pairs of deflector plates l8 and I!mounted in space quadrature.

The electron beam is projected against a target mounted at the other endof the vessel [0, the target comprising a body or sheet 18 of dielectricmaterial, for example mica, having on its rear surface a conductivemember or output electrode l9, for example a metal coating. Closelyadjacent the face of the insulating sheet I8 toward the electron gun isa barrier grid 20, which may comprise a mesh of fine wires or strips 2|.

The deflector plates l5 and I1 areen'ergized from deflecting circuits22, for example to sweep the beam in one direction and to deflect itselectivelyin a coordinate direction. The input signals are applied tothe barrier grid 20, from a source or circuit 23, across a low inputimpedance 24 in circuit with the potentiometer 25, 26. An output voltageis obtained across an output resistor 21 which is associated with aclamp circuit 28.

The device of Fig. 1 as thus far described is generally of knownconstruction and functions to store a signal for one period of time andto reconstruct the signal at a later time. The general operation thereofis as follows: The storage, or removal or reconstruction of a signal isdetermined by conditions extant in the input circuit, the storage beingeffected by alteration of the potential of elements of the surface ofthe dielectric sheet l8 due to impingement of the beam thereon. Forpurposes of simple analysis, consider that a repeating saw-tooth sweepvoltage is applied between one pair of the de flector plates, that inputsignals are applied to the barrier grid only during the odd-numberedtime intervals of the sweep and that during the even-numbered timeintervals the barrier grid is maintained at a constant potential, forexample zero. The clamp circuit is driven in synchronism with the sweepcircuit so that the output resistor is effectively short-circuitedduring the odd-numbered time intervals.

The fundamental signal storage and removal processes will be understoodfrom the following considerations of the phenomenon at an elemental areaof the face of the dielectric Hi toward the gun. Assume that initiallythis element is in equilibrium condition, that is, that no potentialdifference obtains across the face of the dielectric. At the instantthat the beam, in its sweep, impinges upon the elemental area, assumethat because of an input signal the barrier grid is at a positivepotential relative to the dielectric face. Secondary electrons areemitted from this element when the beam impinges thereupon and these aredrawn to the barrier grid. The ratio of secondary electrons leaving theelement to primary electrons reaching it is greater than unity. As aresult of the emission,

the area under consideration will charge positively until it reaches apotential near that of the barrier grid. Thus, as a result of the inputsignal a charge is placed upon the area.

Now, when the beam impinges upon the area during the next time interval,there is no signal upon the barrier grid, as has been noted here tofore,and the area is at a potential higher than that of the grid because ofthe charge thereon. Secondary electrons produced by the impingement ofthe beam upon the area encounter a retarding field so that the ratio ofsecondary electrons leaving the area to primary electrons impinging uponit is less than unity. Consequently the area charges negatively untilits potential approaches that of the barrier grid so that the secondaryelectron current from the area to Y barrier grid is substantially equalto the primary electron current to the area. The change in charge of thearea results in a voltage across the output resistor 2! by virtue of thecapacitance coupling between the front surface of the dielectric l8 andthe electrode I9. This voltage,

as is evident, is representative of the charge placed upon the areaduring the storage period and, hence, of the amplitude of the inputsignal at the time the beam impinged upon the area during the storageperiod.

The principal impedances involved in the operation above described areindicated in Figs. 2 and 3. In the former, for purposes of clarity ofillustration, an elemental area of the front face of the dielectric l8has been shown raised from this surface and is designated as E. This iscoupled, through the dielectric, to the electrode N by a capacitance C1and to the barrier grid by 2. capacitance C2. A third capacitance, C3,obtains between the barrier grid and the electrode IS. The action of theelectron beam in charging the area to a potential approaching that ofthe barrier grid with a signal thereon is analogous to connecting aresistance, shown at R in Fig. 3,

. across the capacitance C2 through a switch S, the

- resistance being established through the imping- Also the outputresistor 21 should be smaller than (:2.

In order to obtain a suitably small value for C3, it has been foundadvantageous to space the barrier grid from the dielectric. It has beenfound, however, that so spacing this grid results in a relatively lowoutput signal level and relatively poor resolution of the input signal.

The reasons for such observed low level and poor resolution and theprinciples involved in this invention for increasing this level andimproving the resolution will be apparent from the following analysiswith reference to Figs. 4A and 4B. In these figures, the primaryelectron beam is represented by the horizontal arrowed lines and has adiameter d adjacent the barrier grid 20 and the dielectric l8, the gridto dielectric spacing being a. As in Fig. 2, for purposes of clarity ofillustration, in Fig. 4A an elemental area E of the dielectric surfaceis represented as raised and in Fig. 4B two such areas, E and E, are sorepresented.

When, as illustrated in Figs. 4A and 4B, the primary beam impinges uponthe elemental area E, secondary electrons are produced, as has beennoted heretofore. Some of these electrons flow to the barrier grid, asindicated by the arrows extending from E tothe grid in Fig. 4A. However,other of the secondary electrons return to the surface of the dielectricbeyond the elemental area E, as indicated in Fig. 4A by the arrowsextending from E to the dielectric 18. Thus, some secondary electronredistribution occurs outside of the area E whereby the charge uponother areas is altered.

Although such redistribution may occur over only a limited area beyondthe area E, that is to a distance to either side of E comparable with orsmaller than the distance 6, the effect thereof may well extend over agreat area. The secondary electrons returning to the dielectric surfacemay be viewed as a spray around the periphery of the area or elementupon which the primary beam impinges. This spray moves with the beam asthe latter sweeps over the surface of the dielectric. Thus, during thestore period, i. e. when the input signal is applied to the barriergrid, a negative space charge follows the beam and alters, specificallyreduces, the charges placed on successive areas or elements of thedielectric surface. Similarly, during the remove" period, i. e. when nosignal is impressed upon the barrier grid, the negative space chargemoving with and in front or ahead of the primary beam, produces analteration in the charge upon successive elemental areas before thecharges are translated into potential variations across the outputresistor 21. Because of these effects, it will be evident that a lowoutput level and poor signal resolution are to be expected.

Consider now the, conditions and relations illustrated in Fig. 4B. Thetwo elemental areas E and E are spaced a distance such that when theprimary electron beam is impinging upon one area, the spray of theresulting secondary electron emission does not reach the other area.Thus, the charge placed upon each area and the translation thereof intoan output signal component is unaffected by the secondary electron sprayfrom the other area. If, furthermore, no emission from the dielectricoccurs from the regions of the dielectric surface between the areas Eand E, deleterious effects of secondary electron distribution heretoforepointed out will be greatly reduced. Hence, a higher output level andbetter signal resolution will be realized.

In accordance with a feature of this invention, the desirable conditionsillustrated in Fig. 4B are produced and a high output level and goodsignal resolution are obtained. In one embodiment, il-

lustrated'in Fig. '1, an auxiliary electrode 29 is provided opposite thebarrier grid, the auxiliary electrode comprising a plurality of parallelwires 30 normal to the direction of the beam sweep and being maintainedat a positive potential by .a source such as a battery 3| so that allsecondary electrons produced by impingement of the beam upon the wires30 will be returned to and collected by these wires. The wires 30 shouldbe spaced from one another to prevent the secondary electron sprayoverlap of successive elemental areas of the dielectric surfaceheretofore noted. For a beam diameter d of 10 mils and a barriergrid-dielectric spacing 5 of 10 mils, wires 30 ten mils in diameter andspaced on 20 mil centers may be used. It will be understood that thewires intercept the primary electron beam and, thus, prevent impingementof the beam upon regions intermediate the areas of the dielectricsurface aligned with the openings between adjacent wires 30, wherebysecondary emission from these intermediate regions is eliminated.

In another embodiment, illustrated in Fig. 6, secondary electrondistribution is reduced by controlling the electron beam. Specifically,the beam is repeatedly blanked during each sweep cycle by applying ablocking bias to the control electrode l2 from a pulsing circuit 32coupled to the sweep circuit 33 by a synchronizing circuit 34 so thatthe elemental areas of the dielectric surface bombarded by the primaryelectron beam are the same for both the store" and remove periods. Thefrequency of the application of the blocking bias is a multiple of thesweep frequency, is greater than twice the signal frequency and suchthat the spacing between adjacent elemental areas bombarded issufficient to prevent the spray overlap heretofore pointed out.

In the embodiment illustrated in Fig. 6, a cylindrical collectorelectrode 35, maintained positive by the source 3|, is provided toreceive any secondary electrons, as from the dielectric or the barriergrid, which pass to the left of the barrier grid in the figure.

.In both the embodiments illustrated in Figs. 1 and 6, the beam isinterrupted, directly or in effect, so that it impinges upon onlyprescribed, spaced elemental areas of the surface of the dielectric 3toward the electron gun. The operation of both embodiments isillustrated graphically in Figs. 5A to 5D, wherein the abscissae aretime, of the same units and with a common zero axis. Fig. 5A shows twosuccessive sweep cycles, one store and one remove; Fig. 5B shows thesignal to be stored or translated, which, as illustrated and heretoforedescribed, is applied to the barrier grid only during the store cycle;Fig. 5C shows the pulsing of the beam, either by blanking of the beam asin the embodiment illustrated in Fig. 6 or by interception by the gridwires 3!] in the embodiment illustrated in Fig. 1; and Fig. 5D shows theoutput signal, composed of pulses, the envelope for which conforms tothe input signal.

The signal resolution also may be improved and the output levelincreased by the creation, adjacent the bombarded surface of thedielectric I8, of a small magnetic field, for example of about 200gauss, normal to this surface. Such a field restricts the area of thesurface to which the secondary electrons constituting the spray, return.The magnetic field may be produced by a cylindrical coil 36 encirclingthe envelope I0, adjacent the dielectric sheet or body l8. It may beused aloneor in combination with beam pulsing, effected either byblanking of the beam as in Fig. 6 or by the grid wires 30 as in Fig. 1.

The invention may be embodied also in a twosided storage device such asshown in Fig. '7. In the device illustrated in Fig. 7, the target I80 isa thin plate of semiconductive material, for example lead glass, and thegun, deflecting system, barrier grid and auxiliary electrode 29 are asin the device shown in Fig. 1 and described hereinabove. The inputsignal to .be stored or translated is applied from the source 23 to thecontrol electrode l2 of the gun. The positive charges produced upon thefront or right-hand surface of the plate I80 as a result of the actionof the beam from the gun leak through the plate to produce correspondingcharges on the rear or left-hand surface of the plate. The charges areremoved, to produce a replica of the input signal in the output re- '9sistor 21 associated with the output electrode 40, by scanning the backsurface with a low velocity beam produced by a cathode 4|, focussedmagnetically by a coil 42 and deflected in coordinate directions byother coils 43.

The device illustrated in Fig. 8 is a modification of those shown inFigs. 1 and 6 and heretofore described. In this embodiment, the inputsignal is applied to the back electrode I9 and the output is taken fromthe collector electrode 35. A secondary current to the collectorelectrode, proportional to the stored signal, is produced during theremove cycles because of the fact that in the discharge process of thebombarded surface of the dielectric, some of the discharge secondaryelectrons reach points beyond the barrier grid 2!], that is to the leftin Fig. 8, and are drawn to the collector electrode. An auxiliaryelectrode 45 maintained negative by a source 46 may be provided betweenthe gun and the collector electrode 35 to prevent secondary electronsfrom reaching deflection plates 11. An advantage of this arrangement isthat the output resistor 21 is not shunted by the capacitance C: betweenthe barrier grid and the back electrode.

Although specific embodiments of this invention have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. A cathode ray device comprising a dielectric target, an electrodeupon one face of said target, an electron gun including a controlelectrode opposite the opposite face of said target for projecting anelectron beam thereagainst, electrode means, in secondary electronreceiving relation with said opposite face, deflection means forsweeping said beam across said opposite face, means for energizing saiddeflection means, and means for controlling the potential of saidcontrol electrode in synchronism with said energizing means torepeatedly blank said beam during each sweep cycle.

2. A cathode ray device comprising a dielectric sheet, a first electrodeupon one. face of said sheet, a barrier grid adjacent the opposite faceof said sheet and parallel thereto, means for projecting an electronbeam against said opposite face, means for sweeping said beam over saidopposite face, means operative upon said beam for restrictingimpingement of said beam during each sweep of said beam over saidopposite face :3 to the same and prescribed elemental areasof saidopposite face, said elemental areas being spaced from each other apreassigned distance, said last-mentioned means including a secondelectrode positioned within said device in the path of said electronbeam, a collector electrode adjacent said barrier grid, an input circuitconnected to said first electrode and an output circuit connected tosaid second electrode.

3. A cathode ray device comprising a target, one face of which issecondary electron emissive, an electrode overlying, spaced from, and insecondary electron receiving relation to said face, means opposite saidface for projecting an electron beam thereagainst, means for deflectingsaid beam to sweep it across said face, and means for reducing undesiredsecondary electron re-distribution over said face as a result ofimpingement of said beam on saidface, said last-mentioned meansincluding an electrode positioned within said device in the path of saidbeam for repeatedly interrupting said beam during the sweep thereofacross said face.

4. A cathode ray device comprising a dielectric body, a conductivemember in contact with one face of said body, electrode means adjacentthe opposite face of said body for receiving secondary electronsemanating therefrom, means for projecting an electron beam against saidopposite face, means for sweeping said beam over said opposite face, andmeans for repeatedly interrupting said beam during the sweep thereofacross said opposite face, said interrupting means comprising anauxiliary electrode having a plurality of beam intercepting partsextending at an angle to the direction of the beam sweep and positionedopposite said opposite face, said beam impinging on only the area ofsaid opposite face between two adjacent beam intercepting parts at anyone time.

5. A cathode ray device comprising a dielectric body, a conductivemember in contact with one face of said body, electrode means adjacentthe opposite face of said body for receiving secondary electronsemanating therefrom, means for projecting an electron beam against saidopposite face, means for sweeping said beam over said opposite face, andmeans for repeatedly interrupting said beam during the sweep thereofacross said Opposite face, said interrupting means comprising circuitmeans associated with said projecting means for repeatedly blanking saidbeam during the sweep thereof.

6. A cathode ray device comprising dielectric target means, electrodemeans on one face of said target means, an electron gun for projectingan electron beam against the opposite face of said target means,electrode means adjacent said opposite face and in secondary electronreceiving relation therewith, means for repeatedly sweeping said beamover said opposite face, and means operative upon said beam forrestricting impingement of said beam during each sweep of said beam oversaid opposite'face to the same and prescribed elemental areas of saidopposite face, said elemental areas being spaced-from each other apreassigned distance, and said beam impinging on only one of saidelemental areas at a time.

7. A cathode ray device in accordance with claim 6 comprising acollector electrode spaced from said opposite face of said target means.

8. A cathode ray device in accordance with claim 6 wherein said meansoperative upon said beamponsistsof an electrode positioned within 9 saiddevice in the path of said electron beam and having electron imperviousportions between said electron gun and said target means and eachaligned with the region between two respective adjacent ones of saidelemental areas.

9. A cathode my device in accordance with claim 6 wherein said meansoperative upon said beam comprises an electrode positioned within saiddevice in the path of said electron beam and means for applying beamblanking pulses to said electrode positioned in the path of saidelectron beam.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Iams Oct. 21, 1941 Hansen Apr. 21, 1942 Burnett Aug. 4, 1942Paumier Oct. 23, 1945 Depp Feb. 25, 1947 Hershberger Dec. 9, 1947 SnyderNov. 23, 1948 Jensen et a1. Apr. 11, 1950

